This invention relates to apparatus for measuring volume of flow of a fluid comprising a casing; a tubular stator which is housed within the casing so that the casing forms a circumferential gallery around it; inlet and outlet ports formed by the casing so that one of them communicates with the gallery and the other with the bore of the tubular stator; a rotor which is journaled within the casing for rotation within the bore of the stator coaxially therewith; oblique slots which are formed in the tubular wall of the stator so that a path for flow of fluid to be measured between the inlet and the outlet ports is formed by the bore of the stator, the oblique slots being orientated so that fluid directed along said path forms a spirally swirling fluid flow which drives the rotor; and means by which a readout indication of the fluid flow volume is derived from rotation of the rotor. Such apparatus will be identified as "fluid flow measuring apparatus of the kind referred to" from here on.
Use of fluid flow measuring apparatus of the kind referred to to provide an indication of a patient's respiration is governed by the sensitivity of the apparatus and by the pressure drop that is established between the inlet and the fluid flow outlet of the apparatus when the apparatus is used. It has long been recognised that that pressure drop needs to be very low if the apparatus is to be useful for indicating the respiratory performance of very sick people. Apparatus which is sufficiently sensitive to respond to flow rates of the order of 2.5 Liters per minute can be used to indicate the respiratory performance of the majority of patients but a greater sensitivity is necessary for measuring the respiratory performance of young children or very sick adults.
One form of fluid flow measuring apparatus of the kind referred to which has been used widely for many years to provide an indication of a patient's respiration is an instrument which is described and illustrated in British Patent Specification No. 765,206 and which is known as a "Wright Respirometer". The port that communicates with the gallery is formed by a tube which projects radially outwards with respect to the stator and the other port is formed by a tube which is coaxial with the stator. That part of the instrument that comprises the rotor, the tubular stator and the casing that forms the gallery, the inlet and the fluid flow outlet is known as a turbine. This instrument will respond to flow rates as low as 2.5 liters per minute so that it can be used to indicate the respiratory performance of most people but it is insufficiently sensitive for it to be useful to measure the respiratory performance of young children or very sick adults.
The use of disposable turbines in a Wright Respirometer has been precluded up to now because of the degree of precision that has been required to produce the instrument so that it will respond to flow rates as low as 2.5 liters per minute without there being unacceptably high pressure drop across it. Hence each such instrument needs to be sterilized prior to its use with each patient.